egl: Add a mutex to _EGLDisplay.

A display may be shared by multiple threads.  Add a mutex for access
control.

src/egl/main/egldisplay.h

@@ -4,6 +4,7 @@
 
 #include "egltypedefs.h"
 #include "egldefines.h"
+#include "eglmutex.h"
 
 
 enum _egl_resource_type {
@@ -53,6 +54,8 @@ struct _egl_display
    /* used to link displays */
    _EGLDisplay *Next;
 
+   _EGLMutex Mutex;
+
    EGLNativeDisplayType NativeDisplay;
 
    EGLBoolean Initialized; /**< True if the display is initialized */